Fig. 35.
Very great care is needed to see that the head of a previously trimmed book is knocked up exactly square before the back is glued, for if it is not, it will be very difficult to get it even afterwards.
ROUNDING AND BACKING
The amount of rounding on the back of a book should be determined by the necessities of the case; that is to say, a back that has, through guarding, or excess of sewing, a tendency to be round, is best not forced to be flat, and a back that would naturally be flat, is best not forced to be unduly round. A very round back is objectionable where it can be avoided, because it takes up so much of the back margins of the sheets, and is apt to make the book stiff in opening. On the other hand, a back that is quite flat has to be lined up stiffly, or it may become concave with use.
Fig. 36.
The method of rounding is to place the book with the back projecting a little over the edge of the press or table, then to draw the back over towards the workman, and, while in this position, to tap it carefully with a hammer (see [fig. 36]). This is repeated on the other side of the book, and, if properly done, will give the back an even, convex form that should be in section, a portion of a circle. Rounding and backing are best done after the glue has ceased to be tacky, but before it has set hard.